The oil and gas industry has developed various tools capable of determining formation fluid properties. For example, borehole fluid sampling and testing tools such as Schlumberger's Modular Formation Dynamics Tester (MDT™) tool can provide valuable information on the type and properties of reservoir fluids in addition to providing measurements of reservoir pressure, permeability, and mobility. These tools may perform measurements of the fluid properties downhole, using sensor modules on board the tools. Alternatively, these tools can withdraw fluid samples from the reservoir that can be collected in bottles and brought to the surface for analysis. The collected samples are routinely sent to fluid properties laboratories for analysis of physical properties that include, among other things, oil viscosity, gas-oil ratio, mass density or API gravity, molecular composition, H2S, asphaltenes, resins, and various other impurity concentrations.
A number of different tools use nuclear magnetic resonance based methods and devices for making measurements in a borehole on fluid samples withdrawn from earth formations. For example, an NMR module on the flow line of the MDT tool may be used for determining different fluid properties from magnetic resonance signals.
Formation fluids often contain several components, each of which likely has a different diffusion property. Therefore, measurement of diffusion coefficients may provide valuable information on the formation fluid properties. Some NMR methods make use of magnetic field gradients to probe the diffusion properties of the formation fluids. For example, some tools are capable of making T1 measurements on fluids flowing in the flowline of a fluid sampling tool. Furthermore, static gradient methods may be used for making diffusion measurements on stationary samples. However, this method of deriving the diffusion data from the NMR measurements assumes a single diffusion constant, despite it being well known that crude oils have a distribution of diffusion coefficients.
Other tools are capable of measuring the relaxation times (T1 and T2) and the diffusion coefficients (D) of fluids in an NMR module of a fluid sampling tool. For example, electromagnets may be used to generate an oscillating pulse field gradient (PFG) in between refocusing pulses of Carr Purcell Meiboom Gill (CPMG) pulse sequence. The oscillating PFG may be phase-locked (synchronized) with the CPMG pulses. The pulse field gradient de-phases the spins and then is turned off for a period, during which the spins diffuse. Following the diffusion period, the oscillating pulse field gradient is turned on to re-phase the spins followed by a spin-echo. The first spin-echo is then re-focused by a train of radio frequency 180-degree pulses to obtain more spin-echoes.